The invention relates to a method of influencing a calender nip in a plant for producing and/or processing an endless, planar sheet-like structure, and to a rotatably mounted roll for implementing this method.
A calender roll of this type is disclosed by DE 44 18 549 A1.
In two-roll and multi-roll systems, cambered rolls are used to compensate for the sag of conventional, cylindrical rolls along the nip and to achieve a uniform pressure distribution over the length of the nip. In this case, the extent of the convexity, also known as the camber, of the roll jacket over the roll length is usually calculated and implemented for a loading case that is defined by specific parameters. It therefore also leads to optimum results only in the case of a use corresponding to the loading case on which the calculation was based, and therefore, in the case of changed conditions, it would have to be possible for this convexity to be matched to these conditions. However, cambered rolls are often solid-core rolls whose jacket is produced, in a complicated process, with the convexity that is defined in accordance with the basic conditions. In the case of such rolls, subsequent adaptation to changed operating conditions is not possible.
Of the various proposals for compensating for the roll sag as a result of the loading in the nip, most were either of limited success or very complicated and were therefore not able to be fully accepted.
The invention is based on DE 44 18 549 A1 cited at the beginning. This document describes a roll, in particular a calender roll, in which the convexity of the roll jacket is set in a deliberate way and, without dismantling the roll, can be adapted to changed operating parameters when at a standstill. For this purpose, the roll is an externally cylindrical hollow body which is welded in a pressure-tight manner between two end disks and is filled with an incompressible medium. In order to produce the convexity, the incompressible medium is put under pressure. For this purpose, closure plugs are provided in both end disks, and are adjustable axially in their guides with the effect of increasing or decreasing the pressure. Although in the case of the known roll such an adjustment in order to change the extent of the convexity can be carried out finely and without dismantling the roll, it is possible only during a machine stop. It is necessary in order to set a pressure distribution that is uniform over the length of the nip and in order to achieve a thickness that is uniform over the width of the planar sheet-like structure. Planar sheet-like structures in the sense of this invention are, so to speak, endless webs made of elastomeric or thermoplastic polymers or webs that are coated with or laminated to such polymers, such as film webs, textile webs or paper webs.
In order to be able to influence the course of the film thickness over the width of the film web during operation, it is known, as described, for example, in DE 35 31 005 C2, to use the machine control system to change the exit geometry of the injection mould as a function of error signals from a thickness measuring system which, continuously or at intervals, samples the film thickness over the width of the film web, with the effect of compensating for the ascertained thickness deviations. However, thickness fluctuations which originate from deviations in the nip width between the calender rolls cannot be prevented in this way.
Given the prior art outlined, the invention is based on the technical object of making it possible to influence and to optimize the nip, which changes under load, in order to achieve a course of the thickness of the extruded sheet-like structure that is uniform over the web width, even during operation.
The invention provides the possibility--for example while maintaining the abovementioned control of the width of the slot-die tool--to make changes to the pressure in the incompressible fluid that produces the convexity (camber) of the roll jacket, and hence fine readjustment of the nip geometry at any time, even during the continuous production or processing of the sheet-like structure. In this case, raising or lowering the pressure in the liquid in order to influence the convexity of the roll jacket is carried out directly or indirectly by raising or lowering the temperature of the hydraulic fluid. This procedure is surprising in the simplicity of its execution, since no relatively great design problems, such as leaks at shaft bushings for a fluid under high pressure, have to be solved.
The temperature control fluid, which is pumped and circulated through the annular space between roll jacket and working face of the roll, is controlled in terms of its temperature and, as a result, the temperature of the fluid which effects the cambering of the roll jacket and is enclosed in the pressure space is raised or lowered. This way of influencing the nip is very effective and beneficial since, in the event of the calender roll being constructed in design terms as a cooling roll, this only needs to be further equipped with suitable sensors and the heating/cooling circuit to be included in a control system which is guided by the machine control system.
In the case of a preferred refinement of the method, the raising or lowering of the temperature of the fluid effecting the cambering is carried out as a function of the thickness of the web, measured continuously or at intervals, including the haul-off speed of the ascertained basis weight and, preferably, at least one further quality feature. The quality feature may be a measured value identifying the surface finish, which is registered by a sensor and fed to the machine control system, where it is compared with stored desired values in order, in the event of impermissible deviations, to derive, in particular, empirically ascertained actuating value changes. Further quality features, which can be registered by suitable sensors, are the stress on the web in the nip, in order to prevent overcalendering of the web, with work markings occurring, or undercalendering of the web as a result of air entrainment because of an excessively wide nip. The pressure prevailing in the nip, or the nip loading, can be ascertained by means of load cells, in order to effect the necessary correction by optimizing the camber of the calender roll. Optical sensors, which operate using polarized light, for example, can finally be used to register mechanical stresses and to provide measured values, which are processed in the machine control system and, in conjunction with the adjustment to the nip, increase the quality of the produced and/or processed product web by optimizing the camber of the roll.
In the case of a further refinement of the method, the ascertained thickness deviations in the product web can be corrected in that the change to the pressure in the incompressible hydraulic fluid by means of temperature control, and the die gap adjustment--as already mentioned above--are combined with each other.
In a refinement of the method, a separate heating/cooling-medium circuit led through the hydraulic fluid is provided in order to change the hydraulic fluid temperature.
In order to be able to correct even thickness deviations occurring locally or outside the central region of the roll by means of deliberate temperature changes in the hydraulic fluid, in the case of a further embodiment, the roll is subdivided over its length into at least two, better at least three, adjoining axial regions, in which the control of the pressure of the respective hydraulic fluid can be carried out independently of the adjacent region.
A preferred device for implementing the novel method results from the development of the roll described in DE 44 18 549 A1. In the case of this roll, the roll jacket of the roll exhibiting the convexity is surrounded concentrically at a distance by a working face of circular ring shape. This is supported on the jacket face by a web fitted on the outer face of the roll jacket. In the case of the known roll, the annular space between the jacket face and the working face, in particular the helical channel formed between the supporting webs, is used to cool the working face, which is heated up by the contact with the product web, for example film web. For this purpose, it is connected to a circulating system for a heating or cooling fluid, which keeps the flow temperature of the heating or cooling fluid to a constant, preset value. The fine regulation of the course of the thickness of the product web over its width is carried out, in the case of the known device, by regulating the nip cross section of the slot die producing the web, specifically as a function of error signals which arise during the continuous measurement of the thickness of the product web over its web width.
It has now been found that a freshly produced film web, when it reaches the calender nip, has already cooled down to such an extent that slight changes to the cooling temperature of the calender roll, these changes being sufficient to achieve the novel pressure change in the incompressible fluid, do not have a severe influence on the quality of the product. Therefore, according to the invention the circulating system that is present in the case of the generic roll for the heating or cooling fluid flowing through the annular space is supplemented by a regulating system by means of which the flow temperature of the heating or cooling fluid flowing through the annular space is set. Regulation is carried out as a function of error signals from the thickness measurement, in conjunction with a quality feature, with the effect of eliminating the thickness deviations by optimizing the camber of the roll jacket.
A further embodiment relates to a roll in a plant which differs from the roll previously described in that the roll jacket which has the convexity is the working face. This means that the annular space is missing here, and therefore so are the circulating system for the heating or cooling fluid flowing through the annular space, in particular the helical channel. In this case, the invention provides for the roll to be equipped with a heating- or cooling-medium circuit, which is used to control the temperature of the incompressible pressure medium, and comprises a heater/cooler, a heat exchanger, a controlled system for controlling the flow temperature of the heating/cooling fluid, and heat-transfer surfaces, by means of which the temperature of the fluid effecting the cambering of the roll can be raised or lowered.
In this case, the heat-transfer surfaces for controlling the temperature of the incompressible pressure medium may be pipes, which are distributed with a radial spacing from the roll axis and end in a pressure-tight manner in the end disks or abut the latter. They run within the cavity in the roll jacket which contains the pressure medium.
In the case of another refinement of the novel roll which is equipped with a roll core that is inserted in a pressure-tight manner into the hollow body of the roll, and whose axial end regions form the bearing journals and in each case reach beyond the axial end disks, it is possible for the heat-transfer surfaces to be constructed in a manner similar to the embodiment previously described. However, in a solution that is modified in design terms, they may also be longitudinal bores which are distributed evenly over the circumference of the roll core, are axially parallel and are arranged at a small distance below the outer face of the roll core.
In order to be able to correct even thickness errors which occur asymmetrically over the width of a product web, in particular a film web, in the case of a development, in particular of the roll surrounded by a cooling jacket, provision is made to subdivide the annular space of the roll jacket and the pressure-medium chamber in the axial direction into a plurality of chambers and to connect them together in pairs, the pairs being connected to respectively associated heat exchangers by associated supply lines.